1. Field of the Invention
This invention relates to the correction of hyperopia, astigmatism, and irregular optical aberrations by changing the shape of the cornea. Specifically, it relates to laser thermal keratoplasty (LTK), where a laser beam is used to heat selected areas of the cornea and cause local shrinkage.
2. Background of the Related Art
Various methods of changing corneal curvature have been developed. In incisional keratotomy, radial, arcuate, or other patterns of incision are made on the corneal surface. These incisions weaken the structural integrity of the cornea and can result in daily refractive fluctuation and long-term refractive shift. Furthermore, surgical errors can result in corneal penetration and intraocular infection.
In mechanical keratomilieusis procedures, a mechanical means is used to remove corneal tissue in the central optical zone. These methods have poor predictability in both the correction of myopia and hyperopia, and can result in severe surgical complications.
In photorefractive keratectomy (PRK), a laser is used to ablate corneal tissue in the central optical zone. A major shortcoming of PRK is the development of haze in the central optical zone. Another shortcoming of PRK is the relatively poor results in the treatment of hyperopia; regression is severe unless a large area of the cornea is ablated. Unlike methods mentioned above, thermal keratoplasty does not involve the undesirable complications relating to incision or excision of corneal tissue.
In thermal keratoplasty, heat is applied to portions of the corneal stroma to produce collagen shrinkage. Corneal stromal collagen is known to shrink to approximately one third of its original length when heated to a temperature range of 60xc2x0 C. to 65xc2x0 C. At higher temperatures, substantial additional shrinkage does not occur, but thermal injury and necrosis may result. Heat can be applied to the cornea with surface probes, penetrating probes, electrical probes, ultrasound, and laser light. Laser heating is the ideal choice because the magnitude, depth and pattern of heating can be more precisely controlled and rapid noncontact application is possible.
Several methods of laser thermal keratoplasty (LTK) have been described. The earliest patent concerning laser LTK, issued to Sand (U.S. Pat. No. 4,976,709) taught the use of optical radiations in the wavelength range of 1.8 to 2.55 microns for the shrinkage of collagen tissues. It also specified the use of laser radiation with corneal-collagen absorption coefficient of 15 to 120 cmxe2x88x921 for keratoplasty. It was taught that a pulsed application of about 100 msec duration is preferable. Sand further taught the use of means for measuring corneal shape before and after application of laser energy to determine the desired and resulting alteration in corneal refraction. It described experiments using circular or linear arrays of circular dots. Sand further taught the use of various chemical agents to reduce the threshold shrinkage temperature of tissue. In the Sand series of patents, block diagrams of a laser delivery system were provided, but no specific optical arrangement or method of operation was described.
U.S. Pat. No. 5,263,951 to Spears, et al. taught the use of a laser delivery system that engages the cornea and produces a variety of irradiation patterns for correcting myopia and hyperopia. The patterns described include patterns of central disk, annular rings, radial lines, and round dots. The patent presents data obtained using a Co:MgF2 laser that produced 0.5 to 2.0 W continuous-wave output. However, in the data presented, much higher powers were used, see, for example, in line 2 of Table C of the patent, 5.3 Diopters of hyperopic correction was produced with an annular pattern with 7 mm outer diameter and 5 mm inner diameter with a fluence level of 1.0 Joules/mm2 delivered in 1.0 second. This translates to a 38 W laser power. Furthermore, the data described was markedly inefficient compared to results described in Moreira, et al., Holmium Laser Thermokeratoplasty, Ophthalmology, Vol. 100, pp. 752-761, 1993.
In Moreira, et al., a similar 6 mm diameter circular treatment pattern using 32 spots of 410 micron diameter and 9 Joules/cm2 produced 7 diopters of hyperopic correction. This means in Moreira, et al. only 1% of the energy used in U.S. Pat. No. 5,263,951 was used, yet resulting in greater refractive correction.
U.S. Pat. No. 5,348,551 to Spears describes an apparatus similar to U.S. Pat. No. 5,263,951, with the difference in that the intended effect of irradiation is keratocyte killing rather than collagen shrinkage. The data presented in the patent show highly variable results in a rabbit study.
U.S. Pat. No. 5,334,190 to Seiler taught the use of a contact laser probe for the delivery of focused laser energy onto the cornea to cause collagen shrinkage. This contact probe limits the irradiation pattern to a series of round dots.
U.S. Pat. No. 5,281,211 to Parel, et al. taught the use of a noncontact laser delivery system for LTK that utilized axicon optics to form a pattern of circular laser spots on the cornea.
In general, the laser delivery systems previously described can be divided into two categories, contact and noncontact. Noncontact delivery is easier to apply, faster and more comfortable for the patient. Of the prior art patents, only U.S. Pat. No. 5,281,211 to Parel, et al. provides an optical system for noncontact delivery. In U.S. Pat. No. 5,281,211, the treatment laser beam is projected simultaneously to a pattern of several treatment areas on the cornea. Consequently, a high laser power output is necessary to achieve the desired corneal stromal heating before significant heat diffusion out of the irradiated area occurs. This precludes the use of diode lasers because diode lasers of suitable wavelengths for LTK currently do not possess the required high peak powers. Nevertheless, diode lasers are attractive light sources because of their low cost, compactness and reliability. A cw InGaAsP/InP diode laser that is capable of emitting 0.5 W at 1.8-1.96 micron wavelength is currently available (SDL-6400, SDL Inc., San Jose, Calif.). This range of wavelength has absorption lengths in water in the range of the typical human corneal thickness, which is highly desirable for LTK. However, a noncontact LTK system that can operate with only 0.5 W of laser power has not been previously described.
It is generally acknowledged that the regression of refractive changes is the main drawback of LTK at this time. The desired refractive change of LTK has been reported to undergo large regression over a period of months.
Despite evolutionary improvements in LTK methodology, significant regression still occurs in all dot patterns of LTK reported so far. The currently available commercial LTK systems from Summit, Sunrise and Technomed all use a ring or concentric rings of laser dot heating, which do not optimally distribute the heating in the corneal stroma to reduce stress concentration. Thus an improved pattern of laser application is needed to optimize the stability of LTK results.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
It is an object, therefore, of the invention to provide both contact and noncontact apparatuses and methods for performing laser thermokeratoplasty that are capable of reducing regression of the intended refractive correction.
Another object of the invention is to provide patterns of laser application for LTK that improve the distribution of tension in the treated portion of corneal stroma and thereby improve the stability of refractive correction.
Another object of the invention is to provide an automated scanning of laser treatment patterns on the cornea for the correction of hyperopia, astigmatism, and irregular optical aberrations of the eye.
Another object of the invention is to provide noncontact apparatus and method of laser thermokeratoplasty that require less laser power than previously described noncontact apparatuses.
Another object of the invention is to provide to provide noncontact apparatus and method of laser thermokeratoplasty that uses a diode laser.
The above objects are achieved by an apparatus for performing laser thermal keratoplasty on an eye of a patient to achieve an intended refractive correction, which includes a treatment laser which outputs a treatment beam; scanning unit for scanning said treatment beam; and controlling and processing units coupled to said scanning means for controlling said scanning unit in a predetermined manner so that said treatment beam is scanned over a treatment area, the shape of said treatment area reducing regression of the intended refractive correction.
The above can be further achieved when said treatment area comprises tapered ends, and/or the treatment area optimizes stress distribution in the cornea of the eye. Some treatment area shapes to achieve this are an oblong shape, a spindle and a crescent shape.
The treatment laser might preferably output a treatment beam having a wavelength such that collagen shrinkage in a majority of the thickness of the cornea occurs. Examples of such laser is a diode laser.
The above and other objects are also achieved by an apparatus for enabling a doctor to perform laser thermal keratoplasty on an eye of a patient, to achieve an intended refractive correction, including: optical beam output unit for outputting a treatment beam and a visible alignment beam aligned to yield a combined beam, dual focusing beams, and a central fixation beam to the eye of the patient, wherein said treatment beam can be selectively turned on or off; beam position adjusting unit for adjusting position of said optical beam output unit with respect to the eye of the patient; optical observation unit for viewing said visible beam, dual focusing beams, and central fixation beams on the eye of the patient, whereby said beam position adjusting unit is adjusted with respect to the eye of the patient until the dual focusing beams intersect so that they appear to be a single spot on the cornea; scanning unit for scanning said treatment beam; and controlling and processing unit coupled to said scanning unit for controlling said scanning unit in a predetermined manner so that said treatment beam is scanned over a treatment area, the shape of said treatment area reducing regression of an intended refractive correction.
The treatment laser is preferably a laser with a wavelength that has an absorption length in corneal tissue that is between 200 to 800 microns. These absorption lengths can be found in the wavelength range between 1.3 to 3.3 microns. The treatment laser can be a cw InGaAsP/InP diode laser which is preferably tuned for emission at approximately 1.87 micron wavelength and more generally in the 1.86-1.89 wavelength range. Other continuous wave and pulsed lasers may also be used. For pulsed lasers, the pulse repetition rate is preferably sufficiently rapid such that the scanned laser spots overlap to form a confluent pattern.
The projection unit preferably comprises collimation optics, optics for focusing laser beams onto the cornea, beam path relay optics, and scanning unit for scanning or steering laser beams to form two-dimensional treatment patterns on the cornea. The steering unit preferably comprises a pair of galvanometer-driven mirrors that control the position of laser beams on the cornea in two orthogonal dimensions.
The apparatus preferably includes unit for projecting a visible laser beam onto the cornea substantially coincident with the position of the treatment laser beam. The aiming laser beam and the treatment laser beam are preferably mixed by using a wavelength-selective mirror. The apparatus preferably also includes unit of projecting visible beams for focusing and unit for projecting a visible beam as a fixation target. Preferably, the unit for controlling the treatment laser projection include a computer with a user input unit enabling the user to control the scanning unit and the output level of the treatment laser which in turn controls the angulation of the treatment and visible beams.
The above and other objects are further achieved by the provision of a method for performing laser thermal keratoplasty on an eye of a patient, to achieve an intended refractive correction, including the steps of: selecting treatment areas based on patient problem information, the shape of which is selected to minimize regression of the intended refractive correction; outputting a treatment beam from a treatment laser; and scanning said treatment beam over said treatment areas.
The above and other objects are further achieved by the provision of a method for performing laser thermal keratoplasty on an eye of a patient, comprising the steps of: outputting a treatment beam and a visible alignment beam aligned to yield a combined beam, dual focusing beams, and a central fixation beam from a beam output unit to the eye of the patient, wherein said treatment beam can be selectively turned on or off; adjusting position of said optical beam output unit with respect to the eye of the patent, said visible beam, dual focusing beams, and central fixation beams on the eye of the patient; viewing said visible beam, dual focusing beams, and central fixation beams on the eye of the patient until the dual focusing beams appear to be a single beam while the patent is affixed to said central fixation beam; and scanning over a treatment area, the shape of said treatment area reducing regression.
According to the present invention, the pattern of corneal irradiation preferably comprises one or more oblong areas of laser heating. The ends of the oblong areas are preferably tapered. The resulting shape may be called xe2x80x9cspindle-shapedxe2x80x9d or xe2x80x9celliptically-shaped.xe2x80x9d If the long axis of the laser treatment area is curved, it is termed xe2x80x9ccrescent-shaped.xe2x80x9d The treatment areas are preferably formed by a pattern of lines of equal width scanned on the cornea.
According to the present invention, a preferred method for hyperopia correction is to use a circular group of radially-oriented spindle-shaped treatment areas placed an equal distance apart. For the correction of regular astigmatism, a preferred method is to use one or more pairs of treatment areas placed at equal distances from the center on the minus cylinder axis of the manifest refraction. For astigmatism correction, the long axes of the treatment areas are preferably oriented circumferentially. For the correction of corneal irregular optical aberration due to local ectasias, the area of irradiation is preferably placed on or near the area of greatest corneal ectasia.
According to the present invention, some of the preferred methods of thermal keratoplasty are performed by using a mask to position a laser probe relative to the eye. In some of these methods, the laser probe comes into contact with the eye.
The above and other objects or advantages and features of the invention will become more apparent from the following description thereof taken in conjunction with the accompanying drawings.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.